General Design Principles for DuPont Engineering Polymers - Module

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Figure 11.54 Tongue-in-groove 0.5 B 0.5 B A B A 0.6 B 0.4 B 0.6 B Dimension A 0.4 mm for B dimensions from 1.5 to 3 mm and proportionally larger or smaller for other wall thicknesses Dimension B General wall thickness Dimension C Optional recess to ensure good contact with welding horn Figure 11.55 Butt joint—variations Strong joints can be achieved with amorphous resins, however, it may be difficult to obtain hermetic seals in complex parts. The main feature of the butt joints is a “V” shaped bead or “energy director” on one of the two mating surfaces which concentrates the energy and limits initial contact to a very small area for rapid heating and melting. Once the narrow area begins to soften and melt, impedance drops and further melting occurs at a faster rate. The plastic in the energy director melts first and flows across the surfaces to be joined. Amorphous plastics have a wide, poorly defined softening temperature range rather than a sharp melting point. When the plastic flows, sufficient heat is retained in the melt to produce good fusion over the entire width of the joint. Delrin ® , Zytel ® , Minlon ® and Rynite ® PET are crystalline resins with no softening before melting and a sharp melting point and behave different than amorphous resins. When the energy director melts and flows across the surfaces, the melt being exposed to C B 1.5 B 10° 10° 60° 102 air can crystallize before sufficient heat is generated to weld the full width of the joint. It is necessary, therefore, to melt the entire joint surface before significant strength can be obtained. (In the case of Zytel ® , exposure of the high temperature melt to air can cause oxidative degradation which results in brittle welds). This phase of the weld cycle is very long as can be seen in Figure 11.56 and 11.57, which are graphs showing typical weld sequences for parts of Delrin ® and Zytel ® using the basic butt joint. The dotted lines indicate the weld time at which an objectionable amount of flash occurs. This flash is a limiting factor in most applications. Beyond this time, results are extremely variable, especially with Zytel ® . Figure 11.56 Butt joint—typical performance, burst pressure vs. weld time Burst pressure, MPa 15 12.5 10 7.5 5 2.5 Delrin ® 500 and 900 F Delrin ® 100 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Weld time, sec Figure 11.57 Butt joint—typical performance, burst pressure vs. weld time Burst pressure, MPa 12.5 10 7.5 5 2.5 Zytel ® 101 (DAM) 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Weld time, sec
) General Part Design The influence of overall part design on ultrasonic welding has not been fully determined. However, some generalizations can be made about certain aspects of part design and their effect on the success of welding. Determining the location at which the welding horn will contact a part is a very important aspect of part design. Some of the considerations for location have already been mentioned in the discussion of the various joint designs. There are two methods of welding, far field and near field as illustrated in Figure 11.58. They refer to the point of horn contact relative to the distance from the joint. Best welding results for all plastics are obtained with near field welding. Therefore, wherever possible, parts should be designed for horn contact directly above and as close to the joint as possible. Figure 11.58 Near field and far field welding Horn Near field Far field In far field welding, the horn contacts the upper part at a distance from the joint and relies on the plastic to transmit the vibrations to the joint. Rigid, amorphous plastics transmit the vibrations to the joint. Rigid, amorphous plastics transmit the ultrasonic energy very well. Although rigid plastics such as Delrin ® , Zytel ® , Minlon ® and Rynite ® PET have a more crystalline structure and can absorb vibrations without creating appreciable heat rather than transmitting them, they are more difficult to weld by the far field technique. Soft plastics such as polyethylene can only be welded by the near field technique. Because they have a high acoustic damping factor, they strongly attenuate the ultrasonic vibrations upon entry into the material. If 103 the joint is too far from the horn, the energy is not transmitted to the joint and the plastic melts at the interface with the horn. Plastics are poor transmitters of shear waves. This fact makes welding more difficult when the geometry of the upper piece is complex. Vibrations are partially attenuated or dissipated at bends, angles or discontinuities such as holes in the structure between the horn and the joint. These features should be avoided. To maximize transmission of vibrations, parts should be designed with a flat contacting surface for the welding horn. This surface should be as broad as possible and continuous around the joint area. Interruptions in contact between the horn and the part may result in weld interruptions. Fillets are desirable for all parts designed for ultrasonic welding. Since the entire structure of both halves being welded is subjected to vibrations, a very high level of stress occurs at sharp internal corners. This frequently results in fracture or sporadic melting. Fillet radii consistent with good molding and structural design practice are suggested. Because of pervasive vibrations, care is suggested when welding parts with unsupported appendages and large spans. Vibrations may be severe enough to literally disintegrate a cantilevered spring, for example, extending from the wall section of a part. Measures, such as rubber lined fixtures or a rubber damper attached to the welding horn, may be taken to dampen such vibrations. This phenomenon can be used to advantage: experiments have shown that molded parts can be degated quickly and with a smooth finish by applying ultrasonic energy to the runners. Ultrasonic Welding Variables The major ultrasonic welding variables are weld time, hold time, pressure and amplitude of vibration. a) Weld Time Weld time is the period during which vibrations are applied. The correct weld time for each application is determined by trial and error. It is important to avoid overwelding. In addition to creating excessive flash which may require trimming, this can degrade the quality of the weld and lead to leaks in parts requiring a hermetic seal. The horn can mar the surface. Also, as was shown in Figure 11.42 melting and fracture of portions of the parts away from the joint area may occur at longer weld times, especially at holes, weld lines, and sharp corners in molded parts. b) Hold time Hold time is a nominal period after welding during which parts are held together and allowed to solidify under pressure without vibrations. It is not a critical
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dDuPont Engineering Polymers Genera
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8—Gears . . . . . . . . . . . . .
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1—General Introduction This secti
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Prototyping the Design In order to
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2—Injection Molding The Process a
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As a general rule, use the minimum
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Figure 3.11 Cored holes Figure 3.12
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Figure 3.19 Mold-ejection of rounde
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• Zytel ® nylon resin—Parts of
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4—Structural Design Short Term Lo
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Form of section Area A d d R R 1 1
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Table 4.03. Formulas for Stresses a
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Form of bar; manner of loading and
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Figure 4.01 Creep Stress (S), MPa (
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Figure 4.05 Creep in flexure of Zyt
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Example 1 It there are no restricti
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Determine the moment of inertia and
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Figure 4.11 Stress curves The compu
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Rim Design Rim design requirements
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Figure 5.08 Chair Seat Zytel ® 71
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Cantilever Springs Tests were condu
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7—Bearings Bearings and bushings
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debris is moved around continuously
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Table 7.01 Coefficient of Friction*
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Figure 7.15: Connecting rod spheric
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8—Gears Introduction Delrin ® ac
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9—Gear Design Allowable Tooth Ben
Page 55 and 56: Delrin ® 500. See the section on D
Page 57 and 58: Figure 9.06 Sizes of gear teeth of
Page 59 and 60: The following additional examples i
Page 61 and 62: In practice, the most commonly used
Page 63 and 64: Driving Gear Meshing Gear Table 9.0
Page 65 and 66: A full-throated worm gear is shown
Page 67 and 68: Worm Material Gear Material Possibl
Page 69 and 70: 10—Assembly Techniques Introducti
Page 71 and 72: For these materials, the finer thre
Page 73 and 74: Table 10.01 Self-Threading Screw Pe
Page 75 and 76: Figures 10.07 and 10.08 show calcul
Page 77 and 78: Undercut Snap-Fits In order to obta
Page 79 and 80: Cantilever Lug Snap-Fits The second
Page 81 and 82: 11—Assembly Techniques, Category
Page 83 and 84: Figure 11.03 Drill spindle position
Page 85 and 86: Inertia Welding By far the simplest
Page 87 and 88: Machines for Inertia Welding The pr
Page 89 and 90: This is not always easy, because ap
Page 91 and 92: The holder a, will have equal and o
Page 93 and 94: Calculations for Inertia Welding To
Page 95 and 96: If, for example, the angles of the
Page 97 and 98: Welding Double Joints The simultane
Page 99 and 100: Figure 11.37 Commercial bench-type
Page 101 and 102: Heat is generated throughout the pa
Page 103 and 104: Figure 11.45 Tapered or stepped hor
Page 105: Weld strength is therefore determin
Page 109 and 110: eticule in the eye piece is suitabl
Page 111 and 112: For this reason, if the strength of
Page 113 and 114: Figure 11.64A shows a variation whi
Page 115 and 116: 3. Vibrations, generated either by
Page 117 and 118: If one part is only vibrated at twi
Page 119 and 120: Another joint design, with flash tr
Page 121 and 122: Figure 11.82. A linear welded motor
Page 123 and 124: B) Commercial linear and angular we
Page 125 and 126: Hot Plate Welding of Zytel ® One o
Page 127 and 128: Figure 11.94 Applications of riveti
Page 129 and 130: Tolerances may be difficult to hold
Page 131 and 132: are used in either hand- or power-o
Page 133 and 134: ather than scrape, these drills wil
Page 135 and 136: The polishing operation is performe
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